1. Field of the Invention
The present invention relates basically to a method and a device for remotely identifying television receivers tuned to a certain television channel and displaying a video signal of this channel. By extension, the method and device of the invention can be used to remotely determine which channel has its video signal displayed through a specific television receiver.
The concept of the invention is to introduce an identification signal in the television signal of the television channel before transmission thereof towards the television receivers, which identification signal is selected and introduced in the television signal so that it is irradiated in the air through each television receiver and through the related equipments connected thereto when such a receiver is tuned to the channel in question and displays the video signal thereof. To irradiate the identification signal, the invention uses the radiation phenomenom inherent to television receivers and caused by the presence of high voltages applied to the CRT (cathode-ray-tube) and modulated by the CRT luminance information at the luminance component frequencies. Such a radiation phenomenom will be discussed in more details in the following description. If the irradiated identification signal is detected in the proximity of a television receiver, it can be determined that this receiver is tuned to the concerned channel and displays the video signal thereof.
A very interesting application of the method and device according to the present invention is for example to detect pirating of secured or scrambled television, such as detection of pirate PAY-TV decoders as will be seen in greater detail hereinafter. Another very interesting possible application is, as an example, regulating control in areas where television license fees are required by law (France, United Kingdom, Europe, etc . . . ) to differentiate between the use of television receivers with computers or VCR's (video cassette recorders), and with received broadcast signals.
2. Brief Description of the Prior Art
Known apparatuses detect the normal electromagnetic radiations from television receivers for remotely determining to which television broadcast station these television receivers are tuned at a given time. For example, the apparatus described in U.S. Pat. No. 2,896,070 (FREMONT et al) granted on July 21, 1959, comprises for this purpose a receiver which detects electromagnetic radiations emitted from the local oscillators of television receivers, and which uses these detected radiations in order to determine to which television broadcast station such receivers are tuned. Another apparatus described in U.S. Pat. No. 2,903,508 (HATHAWAY) granted on Sept. 8, 1959, also comprises a receiver adapted for remotely determining to which television broadcast station the television receiver of a residence is tuned, by comparing the phase of a synchronizing signal obtained from the video signal emitted by a television broadcast station with the phase of another similar signal irradiated from components of the television receiver in question.
The disadvantage of such prior art apparatuses is that they are limited to the analysis of the normal electromagnetic radiations from the television receivers, which analysis requires substantially complicated circuits and consequently reduces the reliability of these known apparatuses.